Implantable device

ABSTRACT

An implantable device comprising: a stepper motor; a moveable member, moveable by the stepper motor; and an oscillator, wherein the oscillator is influenced by a signal derived from or supplied to the stepper motor to enable information on the moveable member to be fedback to an external controller by passive telemetry.

The present invention relates to an implantable device comprising amoveable member for use in medical control and/or regulation.

Various types of long-term medical implantable devices using an actuatorhave been developed over the years, such as drug delivery pumps orregulation devices for the treatment of obesity of urinary incontinence.These devices are required to perform a specific action, for exampledelivery systems have to diffuse a drug at a precise location in thebody and regulation devices have to apply greater or lesser degrees ofconstriction to an organ or a vessel. The actuator typically comprises adriving unit for providing motive force, and a movable member driven bythe driving unit to perform the action. A number of different physicalprinciples have been employed in such actuators, depending on the typeof action, for example:

-   -   electromagnetic force—used by all types of electric motors used        in peristaltic pumps or in valves;    -   pneumatic force—in drug delivery systems a pressurised gas        pushes a compressible medication reservoir to expel the drug        through a specific fluidic resistance; and    -   hydraulic force—in regulation devices, constriction is achieved        by an annular balloon filled with a physiologically acceptable        solution.

It is also necessary for the action to be precisely controlled and theusers need to be confidant that the action is correctly performed.Different types of action control are known:

Firstly the control can be performed autonomously by the device itself,and this may be either active or passive. An example of active controlis where the control is electronic and performed by a microprocessor. Inprogrammable infusion devices a peristaltic pump is driven by amicroprocessor which controls the number of electrical pulses deliveredto the pump. All the activity of the pump is recorded in a memory andthe patient can access the data and change the pump parameters by radiofrequency (RF) communication with an external control unit. An exampleof passive control is in some drug delivery devices which usepressurised gas to compress a medicament reservoir, the stability of theflow is determined by the design of the output fluidic resistance. Thedevice is tested and calibrated for different types of drugs during themanufacturing process.

Secondly the control may be done by using an external diagnostic system,such as an Echo-Doppler for flow measurement or an angiography system tocheck the restriction size of a gastric banding.

There are a number of problems and drawbacks with the above existingimplanted actuator technologies:

-   -   In programmable infusion pumps, the motor is powered by battery.        The battery has a finite lifetime which means it needs to be        changed, which involves additional surgery. The encapsulation of        the battery also has to be perfect to avoid harmful leaks and        diffusion. If there is a failure with the electronics, the pump        could start on its own, because electric power is available in        the device.    -   In infusion pumps with passive fluidic regulation, flow rate is        affected by changes in altitude and/or temperature. The        viscosity of the infusion solution, as well as the arterial        pressure at the location of the catheter tip in vascular        applications, can also affect flow rate.    -   In gastric regulation devices, an access port is located        subcutaneously to regulate the amount of liquid in the hydraulic        circuit. This regulation can be done only by puncture through a        silicone membrane. This means that there are risks of infection        and leakage.    -   Typically there is no direct feedback on whether the function to        be achieved by the actuator has been fulfilled. The only way to        get this information is indirectly, for example by measurements        of the effects e.g. using an external diagnostic system.

To alleviate some of these problems there have been proposals totransmit energy to an implanted device using RF waves. There has alsobeen proposed a method of getting data back from an implanted deviceknown as “passive telemetry by absorbtion modulation” devised by Dr. P.A. Neukomm in the late 1980s, see for example CH 676164, WO 89/11701, EP0377695 and the article Passive Wireless Actuator Control and SensorSignal Transmission, Sensors and Actuators, A21-A23 (1990), 258-262.According to this method there is no need for an RF emitter in theimplanted device, which means that no battery is used in the implant.

However, there are still a number of existing problems, such as thecomplexity and additional components needed to detect the status of theactuator, convert this to a signal for feedback and to achieve thedesired modulation.

It is an object of the present invention to alleviate, at leastpartially, some or all of the problems previously described.

Accordingly the present invention provides an implantable devicecomprising:

-   -   a stepper motor;    -   a moveable member, moveable by the stepper motor; and    -   an oscillator,    -   wherein the oscillator is influenced by a signal derived from or        supplied to the stepper motor to enable information on the        moveable member to be fedback to an external controller by        passive telemetry.

Using the signal derived from or supplied to the stepper motor toinfluence the oscillator means that the feedback of information can bedone directly without requiring processing by a microprocessor in theimplant. The use of a stepper motor is particularly advantageous becauseof the simplicity of the supply circuitry and because the signalsapplied to its coils are directly related to the displacement of themovable member; the displacement is proportional to the number of pulsesgiven to the motor coils. Therefore it is not necessary to add shaftencoders or sensors to determine the status of the actuator.

Preferably the signal is the electrical signal applied to one coil ofthe stepper motor, or alternatively the signal is the voltage induced ina secondary coil wrapped around a coil of the stepper motor.

Preferably the signal modifies the frequency of the oscillator. Forexample, where the oscillator is a voltage controlled oscillator (VCO),this can be done directly, thereby eliminating the need for additionalcomponents and modulation means.

Preferably the device further comprises a microcontroller for drivingthe stepper motor, wherein the oscillator also comprises the externaloscillator for providing a clock signal to the microcontroller. By usingthe external oscillator of the microcontroller as part of the oscillatorfor performing the modulation of the feedback signal, no additionalcomponents are necessary, and in particular it is not necessary toprovide two dedicated oscillators.

Preferably a detector is provided to detect a reference position of themovable member and to influence the oscillator accordingly, preferablyby causing a frequency shift of the oscillator. In this way, a smallimprecision, such as an offset, in the calculated position of themovable member can be corrected.

The present invention also provides a system comprising: an implantabledevice as defined above; and an external controller comprising means forcounting pulses in said signal fed back by passive telemetry todetermine the motion of the stepper motor and the position of themovable member. The number of pulses in the signal contains informationon the number of steps done by the rotor, and therefore provides an easyand precise way for determining the position of the movable member.

Preferably the external controller further comprises means for analysingthe shape of the feedback signal to detect blockage of the steppermotor. Even if pulses are applied to the coils of the stepper motor, ifthe stepper motor cannot move because of a blockage, for examplesomething obstructing the movable member, the rotor of the stepper motorwill not turn and the magnetic circuit is disturbed. Consequently, byinduction, the shape of the feedback signal is affected and suchperturbations of the signal shape can be detected by the externalcontroller.

Preferably the device or system described above further comprises oneselected from the group consisting of:

-   -   a flow controller adjustable by said moveable member for blood        flow regulation on native vessels or artificial grafts;    -   gastric banding adjustable by said moveable member for treatment        of obesity;    -   oesophageal banding adjustable by said moveable member for        treatment of Gastro Enteral Reflux Disease;    -   an artificial sphincter adjustable by said moveable member for        treatment of urinary incontinence;    -   an artificial sphincter adjustable by said moveable member for        treatment of faecal incontinence;    -   an artificial sphincter adjustable by said moveable member for        use following a colostomy;    -   an artificial sphincter adjustable by said moveable member for        use following an ileostomy; and    -   a drug infusion system adjustable by said moveable member.

The present invention also provides use of a device or system as definedabove in the manufacture of a medical device for use in at least oneselected from the group of:

-   -   blood flow regulation on native vessels or artificial grafts;    -   gastric banding for treatment of obesity;    -   oesophageal banding for treatment of Gastro Enteral Reflux        Disease;    -   control of an artificial sphincter for treatment of urinary        incontinence;    -   control of an artificial sphincter for treatment of faecal        incontinence;    -   control of an artificial sphincter following a colostomy;    -   control of an artificial sphincter following an ileostomy; and    -   control of a drug infusion system.

The present invention also provides use of a device or system definedabove in an application selected from the group consisting of:

-   -   blood flow regulation on native vessels or artificial grafts;    -   gastric banding for treatment of obesity;    -   oesophageal banding for treatment of Gastro Enteral Reflux        Disease;    -   control of an artificial sphincter for treatment of urinary        incontinence;    -   control of an artificial sphincter for treatment of faecal        incontinence;    -   control of an artificial sphincter following a colostomy;    -   control of an artificial sphincter following an ileostomy; and    -   control of a drug infusion system.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:—

FIG. 1 is a schematic illustration of a passive telemetry systemcomprising a wireless powered implanted device and external controller;

FIG. 2 is a block diagram of a passive telemetry system using theprinciple of absorption modulation;

FIG. 3 is a block diagram of a passive telemetry system using theprinciple of FM-AM absorption modulation; and

FIG. 4 is a block diagram of a passive telemetry system embodying thepresent invention.

In the figures, like reference numerals are used to identify like items.

Some principles of passive telemetry will firstly be explained to assistunderstanding of the present invention.

FIG. 1 shows the basic system used by the invention comprising anexternal controller 100 connected to an external antenna loop 102, andan implantable device 104 implanted beneath the skin 106 of the patient.The device 104 includes a receiving antenna, control electronics and anactuator. The external controller 100 delivers RF energy to the externalantenna loop 102 which is transmitted to the implanted device to powerits electronics and actuator. The external controller 100 alsodemodulates feedback information from the implantable device 104.

Referring to FIGS. 2 and 3, in the external controller, an RF generator1 produces a forward-flowing wave at a carrier frequency f_(c), forinstance 27 MHz. The external loop antenna 3 emits mainly a magneticfield which is coupled into the implanted loop antenna 4. The antennas3, 4 have inductancies L_(pp) and L_(ps), and are tuned with capacitorsC_(sp), C_(pp) and C_(ps), C_(ss) in order to have high efficiency in RFpower transmission. This efficiency depends on the resonance frequencyof the antennas and on the impedance matching between the emitter andthe receiver. The RF energy is then converted into DC electrical energyby the RF-DC converter 5 and the DC energy is used to supply theelectronics, actuator and so on in the implanted device 104, which inFIG. 2 this is represented by the load R_(L) 6. Working instructions canbe superimposed on the RF signal by amplitude modulation (AM).

The maximum transmission distance between the antennas 3, 4 depends ontheir diameters, for example it might be in the region of 4 cm for thecase of an external loop antenna 3 of diameter 6 cm and an implantedloop antenna 4 of 2 cm diameter.

The passive telemetry for feedback of information from the implanteddevice 104 to the external controller is carried out on the same RFfields as that which transfers energy to the implanted device, bymodulation of the absorption Tate within the implanted device. Theprinciple is based on detuning of the coupled antennas 3, 4. The signalS to be transmitted back is used to vary the load within the implanteddevice 104 supplied by the RF-DC converter 5, so that a changing amountof RF energy is absorbed. The matching of the antennas 3, 4 varies atthe same time, therefore the amplitude of the reflected wave changes inthe external controller. By decoding the reflected wave, a signal S′,proportional to the signal S can be extracted back in the externalcontroller. Two different methods of coding the signal S exist:

1. Amplitude modulation (AM) of the reflected wave. As illustrated inFIG. 2, the signal S directly influences the load R_(L) 6 such that theenergy absorption rate is proportional to S and the amplitude of thereflected wave contains the information on signal S. In the externalcontroller, the directional coupler 2 separates out the reflected waveto obtain the signal S′ which is proportional to desired signal S. Thismethod is simple, but is sensitive to any variation in the quality ofcoupling between the antennas 3, 4.

2. Modulation of the frequency of the amplitude modulation (FM-AM). Asshown in FIG. 3, the signal S is applied to a voltage controlledoscillator (VCO) 8 such that the signal S is converted linearly into anoscillating signal at the frequency Fs, where Fs equals k×S. The signalFs drives a switch 7 such that during the ON state of the switch 7 thereis an increase in energy absorption from the RF-DC converter 6.Therefore the absorption rate is modulated at the frequency Fs and thusthe frequency of the amplitude modulation of the reflected wave containsthe information on the signal S. In the external controller, thedirectional coupler 2 separates the reflected wave where it can bedecoded by FM demodulation in the demodulator 9 to obtain the signal S′.This method allows the transmission of different signals carried atdifferent frequencies. It also has the advantage that the ON state ofthe switch can be very short and the absorption very strong withoutinducing an increase in average consumption and therefore the feedbacktransmission is less sensitive to variation in the quality of couplingbetween the antennas 3, 4.

The presently preferred embodiment of the invention is illustratedschematically in FIG. 4 and uses the principle of passive telemetry byFM-AM absorption modulation. The external controller on the left handportion of FIG. 4 includes a microprocessor 10 with a user interfacecomprising a keyboard 10 a and display 10 b, which produces a signalcomprising one or more data bytes to be transmitted to the implantabledevice 104. FIG. 4 explicitly shows a modulator 11 for amplitudemodulation of the RF wave from the RF generator 1 which is emitted bythe antenna 3. The emitted wave is received by the antenna 4 in theimplantable device 104 and the AM demodulator 12 extracts the data bytesfrom the envelope of the RF signal and they are decoded and written intoan EEPROM of the micro controller 13. A special code is used whichallows an easy decoding by the micro controller, but also providesmaximal security against communication failure. The micro controller 13is provided with an external oscillator 14 for providing a clock signalto the micro controller 13. Various different embodiments exist for theexternal oscillator 14, for example:

-   -   A relaxation oscillator consisting of an external        resistor-capacitor network connected to a discharging logic        circuitry already implemented in the micro controller. This        simple solution with only two additional components is suitable        when the stability of the frequency is not so important. Another        advantage is low current consumption.    -   A crystal oscillator consisting of a resonant circuit with a        crystal, capacitors and logic circuits. This solution is        preferable when a stable frequency is needed, but the number of        external additional components and the current consumption are        greater.

In the present invention the embodiment of the oscillator with externalRC network is preferable, because of its simplicity.

The micro controller 13 interprets the received instructions to producean output for driving the actuator 15, for example to produce motion ina particular direction by a particular amount. The actuator 15 comprisesa bi-directional stepper motor and a movable member, movable by thestepper motor. A stepper motor is very suitable for use in animplantable device 104 because of its small thickness, typically 2 mm,its low power consumption, typically 25 mW, and its output torque afterreduction gearings, typically 3 mNm. The stepper motor can produce veryprecise movement of the movable member, and depending on the particularapplication, this may be either rotational or axial movement, such thatthe mechanical output of the actuator is either a rotational torque oran axial force.

The stepper motor of the actuator 15 does not require complicated supplycircuitry. Its two coils are directly connected to the micro controller13, which receives the working instructions from the demodulator 12,interprets them and provides the voltage sequences to the coils. Whenthe supply of voltage pulses to the stepper motor stops, all gearingsstay in their position, even if a reverse torque or force is applied tothe movable member of the actuator 15.

Using the stepper motor of the actuator 15 it is possible to obtaininformation on the movable member without adding sensors or encoders,because the displacement is proportional to the number of pulses givento the stepper motor coils. Two signals ensure precise control:

1. S_(A), the actuator signal. According to one embodiment, this signalS_(A) is the voltage signal taken at one of the outputs of the microcontroller 13 connected to the motor coils. According to anotherembodiment, this signal S_(A) is the induced voltage on a secondary coilwrapped around one motor coil of the actuator 15. In either case, thispulsating signal contains the information of the number of steps done bythe rotor and also the indication of a blockage of the mechanism. If therotor of the stepper motor does not turn, the magnetic circuit isdisturbed, and by induction, it affects the signal S_(A), for examplealtering the shape of the signal S_(A). This can be detected in theexternal controller, as will be described later. The signal S_(A) couldequally be derived from the current applied to a motor coil instead ofthe voltage.

2. S_(RP), the reference position signal. The actuator 15 includes adetector, such as an electrical contact switch, a Hall-effect switch, aforce-sensing resistor, a variable inductor, or a piezoresistiveelement, which is activated when the movable member of the actuatorreaches a reference position. This information can also be used by theexternal controller, as will be described below.

The signals S_(A) and S_(RP) are converted into frequencies through theexternal oscillator 14 which is a voltage controlled oscillator (VCO).The voltage level of the signal S_(A) is applied to the externaloscillator 14 to vary its frequency F_(osc) proportionally to the signalS_(A). Thus F_(osc) contains all the information of S_(A). When themovable member is as the reference position, the detector describedabove is activated to produce the reference position signal S_(RP) whichis used to induce a constant shift of the frequency F_(osc), which shiftis easily distinguishable from the variations due to signals S_(A). If arelaxation oscillator is used as oscillator 14, the signals S_(A) andS_(RP) modify the charging current of the external resistor capacitornetwork. Preferably, the relaxation oscillator consists of an externalresistor-capacitor network connected to a transistor and a logic circuitimplemented in the micro controller circuitry. With S_(A) and S_(RP),the goal is to modify the charging current of the capacitor of the RCnetwork in order to change the frequency of the relaxation oscillator.If the charging current is low, the voltage of the capacitor increasesslowly and when the threshold of the transistor is reached, thecapacitor discharges through the transistor. The frequency of thecharging-discharging sequence depends on the charging current. If acrystal oscillator is used as oscillator 14, S_(A) and S_(RP) modify thecapacitor of the resonant circuit. Preferably the crystal oscillatorcircuit consists of a crystal in parallel with capacitors. The crystaland capacitors form a resonant circuit which oscillates at a fixedfrequency. This frequency can be adjusted by changing the capacitors. Ifone of these capacitors is a Varicap (kind of diode), it is possible tovary its capacitance value by modifying the reverse voltage applied onit. S_(A) and S_(RP) can be used to modify this voltage.

Hence, the signals S_(A) and S_(RP), are used to modify at least oneparameter of a resistor-capacitor (RC) network associated with theoscillator 14 or at least one parameter of a crystal oscillatorcomprising the oscillator 14.

As can be seen in FIG. 4, the signals S_(A) and S_(RP), derived from thestepper motor or from the output of the microcontroller 13, can be useddirectly for frequency modulation by the VCO 14 without any encoding orintervention by the microcontroller 13. By using the external oscillator14 of the micro controller 13 as part of the VCO for the feedbacksignal, no additional components have to be added, and the operation ofthe micro controller 13 is not adversely affected by the changes in theoscillator frequency F_(osc). The oscillating signal F_(osc) drives thevoltage driven switch 7 for absorption modulation, such that feedbacktransmission is performed with passive telemetry by FM-AM absorptionmodulation, as previously described.

In the external controller, the feedback signal F_(osc) is detected bythe pickup 16 and fed to FM demodulator 9 which produces a voltageoutput V_(OUT) which is proportional to F_(osc). V_(OUT) is fed tofilter 17 and level detector 18 to obtain the information correspondingto the actuator signal S_(A) which corresponds to the pulses applied tothe stepper motor coil. The microprocessor 10 counts these pulses andcalculates the corresponding displacement of the movable member of theactuator 15, which is proportional to the number of pulses. The signalV_(OUT) is also passed through analogue-to-digital converter 19 and thedigital output is fed to the microprocessor 10 where signal processingis performed to detect perturbations of the shape of the feedback signalwhich would indicate a blockage of the rotor of the stepper motor whichwould indicate, for example, an obstruction of the movable member. Themicroprocessor 10 would stop counting any detected motor pulses when itdetected that the actuator was blocked, and would output an indicationof this status. The level detector 20 produces an output when it detectsthat the demodulated signal V_(OUT) indicates the presence of thereference position signal S_(R)P due to activation of the referenceposition detector. This output induces a reset of the position of themovable member calculated by the microprocessor 10 in the externalcontroller. In this way, a small imprecision, e.g. an offset, can becorrected.

As shown in FIG. 1, and by the dashed line in FIGS. 2, 3 and 4, theimplantable device 104 contains all the electronics and the actuatorencapsulated into a compact biocompatible plastic package, which allowsgood RF transmission to the internal antenna loop 4. However, accordingto alternative embodiments of the invention, the antenna loop 4 and/orthe actuator 15 may be provided remotely from the rest of the device,for example to enable the antenna coil 4 to be nearer the skin of thepatient for better transmission, or to enable the actuator 15 to beimplanted at a deep location.

The device embodying the the invention described above can be used forany suitable application, in which the lumen of any bodily vessel ortube (native or artificial) should be adjusted, for example: blood flowregulation on native vessels or artificial grafts, gastric banding fortreatment of obesity, oesophageal banding for treatment of GERD (GastroEnteral Reflux Disease), control of an artificial sphincter fortreatment of urinary incontinence, control of an artificial sphincterfor treatment of faecal incontinence, control of an artificial sphincterfollowing a colostomy, control of an artificial sphincter following anileostomy. The technology is also suitable for use in drug infusionsystems.

1. An implantable device comprising: a stepper motor; a moveable member,moveable by the stepper motor; and an oscillator, wherein the oscillatoris influenced by a signal derived from or supplied to the stepper motorto enable information on the moveable member to be fedback to anexternal controller by passive telemetry.
 2. A device according to claim1, wherein said signal is the electrical signal applied to one coil ofthe stepper motor.
 3. A device according to claim 1, wherein said signalis the voltage induced in a secondary coil wrapped around a coil of thestepper motor.
 4. A device according to claim 1, wherein the oscillatordrives an absorption modulator for use in feedback of said informationby passive telemetry using FM-AM modulation.
 5. A device according toclaim 1, wherein said signal modifies the frequency of the oscillator.6. A device according to claim 5, wherein said signal is used to modifyone of: at least one parameter of a resistor-capacitor networkassociated with the oscillator; and at least one parameter of a crystaloscillator comprising the oscillator.
 7. A device according to claim 1,further comprising a microcontroller for driving the stepper motor,wherein the oscillator also comprises the external oscillator forproviding a clock signal to the microcontroller.
 8. A device accordingto claim 1, wherein a reference position of the moveable member isdetected by a detector which is used to influence the oscillator.
 9. Adevice according to claim 8, wherein said detector causes a shift infrequency of said oscillator when the reference position is detected.10. A device according to claim 8 wherein the detector is selected fromthe group consisting of: an electrical contact switch, a Hall-effectswitch, a force-sensing resistor, a variable inductor, and apiezoresistive element.
 11. A device according to claim 1, encapsulatedinto a biocompatible, non-metallic package.
 12. A system comprising: animplantable device according to claim 1, and an external controllercomprising means for counting pulses in said signal fedback by passivetelemetry for determining the motion of the stepper motor and theposition of the moveable member.
 13. A system according to claim 12,wherein said external controller further comprises means for analysingthe shape of said signal to detect blockage of the stepper motor.
 14. Adevice according to claim 1, further comprising one selected from thegroup consisting of: a flow controller adjustable by said movable memberfor blood flow regulation on native vessels or artificial grafts;gastric banding adjustable by said moveable member for treatment ofobesity; oseophageal banding adjustable by said moveable member fortreatment of Gastro Enteral Reflux Disease; an artificial sphincteradjustable by said moveable member for treatment of urinaryincontinence; an artificial sphincter adjustable by said moveable memberfor treatment of faecal incontinence; an artificial sphincter adjustableby said moveable member for use following a colostomy; an artificialsphincter adjustable by said moveable member for use following anileostomy; and a drug infusion system adjustable by said moveablemember.
 15. Use of a device or system according to claim 1 in themanufacture of a medical device for use in at least one selected fromthe group of: blood flow regulation on native vessels or artificialgrafts; gastric banding for treatment of obesity; oseophageal bandingfor treatment of Gastro Enteral Reflux Disease; control of an artificialsphincter for treatment of urinary incontinence; control of anartificial sphincter for treatment of faecal incontinence; control of anartificial sphincter following a colostomy; control of an artificialsphincter following an ileostomy; and control of a drug infusion system.16. Use of a device or a system according to claim 1 in an applicationselected from the group consisting of: blood flow regulation on nativevessels or artificial grafts; gastric banding for treatment of obesity;oseophageal banding for treatment of Gastro Enteral Reflux Disease;control of an artificial sphincter for treatment of urinaryincontinence; control of an artificial sphincter for treatment of faecalincontinence; control of an artificial sphincter following a colostomy;control of an artificial sphincter following an ileostomy; and controlof a drug infusion system.
 17. A system according to claim 12, furthercomprising one selected from the group consisting of: a flow controlleradjustable by said movable member for blood flow regulation on nativevessels or artificial grafts; gastric banding adjustable by saidmoveable member for treatment of obesity; oseophageal banding adjustableby said moveable member for treatment of Gastro Enteral Reflux Disease;an artificial sphincter adjustable by said moveable member for treatmentof urinary incontinence; an artificial sphincter adjustable by saidmoveable member for treatment of faecal incontinence; an artificialsphincter adjustable by said moveable member for use following acolostomy; an artificial sphincter adjustable by said moveable memberfor use following an ileostomy; and a drug infusion system adjustable bysaid moveable member.